This invention pertains to passive-type range determining systems for measuring the distance (i.e., range) to a source of electromagnetic radiation, especially in the radio frequency spectrum.
As described in a U.S. Pat. No. 4,339,755, issued to James M. Wright on July 13, 1982, and assigned to The Boeing Company of Seattle, Washington, there are certain navigational and surveillance operations in which it is desirable to measure the range between an antenna platform and a distant source of electromagnetic radiation of unknown position, without transmitting from the platform, a signal, such as a radar signal, that would reveal the location of the platform. The platform may, for example, be a manned or unmanned aircraft.
As described in U.S. Pat. No. 4,339,755, signal monitoring equipment is carried by the aircraft platform for determining the range to the emitter by first measuring a small time differential t.sub.21 that occurs between the receipt of the emission at one scanning antenna carried on the platform relative to the time of receipt of the same emission by a second scanning antenna mounted on the platform at a predetermined baseline separation from the first-mentioned antenna. The scanning of the antennas is synchronized so that their respective beams are parallel and sweep across the emitter at slightly different times. The time differential t.sub.21 together with signals representing an angle of arrival .phi. of the radiation, a distance L of the baseline separating the antennas, and their scanning rate .omega..sub.s are processed in a ranging formula: EQU R=(Lcos .theta.)/(t.sub.21 .omega..sub.s)
In a disclosed embodiment in U.S. Pat. No. 4,339,755, the pair of scanning antennas are mounted on the opposed wing tips of an aircraft in order to achieve a baseline length L that is as long as practical for the given platform, to improve the accuracy of the range measurement. Nevertheless, even with such a baseline separation, the accuracy of this existing arranging system has certain limitations. Primary limitations are the beam widths of the scanning antennas and the relatively large inertial constraints on the mechanical rotation of the antennas. Size constraints on the individual antennas carried by the aircraft result in a beam width and hence directional sensitivity that may be too large for precision ranging under some conditions. The beam width is especially limiting when there is a need to cover a relatively wide spectrum of frequencies. For example, at the low end of a typical frequency spectrum of from 300 MHz to 30 gigahertz, the 1 meter wavelength (at 300 MHz) would require an antenna having a span of approximately 200 feet to collimate the beam width down to 1.degree.. At the high end of such range, a wavelength of 1 cm (at 30 gigahertz) would require an antenna span of approximately two feet, which in itself, might be feasible, but the difficulty of electromechanically synchronizing the scanning of antennas of this size would be very difficult and significantly limit the ranging accuracy.
In the case of scanning antennas mounted on an aircraft, for example, on the opposed wing tips, flexure and vibration of the aircraft body can cause significant and heretofore uncorrectable phase aberrations in the axes of the antenna beams and, hence, phase errors in the signals received from a distant emitter. Accordingly, any attempt to enhance the gain of the scanning antennas, i.e., narrow the beam width in order to improve the theoretical ranging accuracy, will encounter limitations in the signal resolution due to uncorrected, spurious phase shifts of the antenna signals due to small but significant relative movement of different points on the aircraft wings and body. Hence, the lack of physical stability or rigidity of the aircraft platform on which the scanning antennas are mounted, places a limitation on the degree of improvement that can be achieved in ranging resolution by sharpening (narrowing) the beam width of each of the scanning antennas.
Furthermore, in prior ranging and direction of arrival measurement systems that rely on mechanical scanning of one or more antennas, the relatively slow scanning rate may fail to pick up transient emissions that can occur while the antenna is moved away from the source.